In displaying coded information by means of, for example, a printer, CRT display device, etc., the image area is typically divided into a plurality of blocks into which characters or symbols may be displayed. On a normal full page, for example, there are typically 102 blocks or character boxes in each of 66 vertically displaced rows. In displaying characters or symbols on a typical CRT display each character box might hold a character or symbol consisting of 128 pels, or individual white or black picture elements, arranged in an 8.times.16 array. In displaying characters and symbols with a typical ink jet printer, the character boxes might contain 800 pels arranged in a 20.times.40 array. Each of these pel positions in a character box is filled in conjunction with the other pels according to one of, typically, 256 patterns in a font. The number of the pattern is derived from the character code associated dynamically with a particular character box; hence, the term "coded information".
The simplest, and by far the most common, method of storing these collections of patterns is to store one bit in memory for each possible pel location in the character box. Addressing a pattern in memory is then simply a matter of multiplying the number of the symbol by the amount of memory required to store each symbol to "skip over" all lower numbered symbols and point to the symbol of interest.
With a high resolution printer, for example, a 350 pel per linear inch printer, the amount of storage required for even a single font can grow very large. In the prior art it has been proposed to conserve memory by storing only the pels in a character box actually covered by a symbol. A period, then, requires less memory than an "X". The prior art system then addresses and places symbols indirectly according to the following algorithm summarized to include points relevant to the inventive concept herein. In this prior art system the character number points to a offset table, for which a pointer to the top of the symbol in the font table is read. With the symbol is stored a number indicating how much of the font storage is applicable to that character or symbol. Another number stored with each coded symbol in the text stream tells a position on the page at which display of the character or symbol is to start, distinguishing between a comma and an apostrophe which both have the same height but start at different levels on the page. Assuming one byte is required for the starting level, one byte for the height, and two bytes for the offset, and assuming 256 symbols in the font and 4000 character positions on the page, the system uses almost 5000 bytes for the pointers. In large systems this use of 5000 bytes for pointers represents a relatively small cost in relation to the system cost as a whole. However, in a small font for a CRT display, the pointers would occupy a space comparable with that saved by the font table compression. Thus the storage of blank space would be supplanted with storage of pointers to provide no net gain.
It would, therefore, be highly advantageous to provide a technique of font compression for storage and decompression for display requiring less memory per font to allow for the storage of more characters and symbols in a memory of limited size.